With the observation angle of TFT-LCD increasing, the image contrast has been reduced continuously, and the image definition has been decreased. This is due to the birefringence of liquid crystal molecules in the liquid crystal layer changing along with the observation angle. Compensating with a compensation film of a wide viewing angle can reduce light leakage of the dark state image effectively, and increase the image contrast significantly in a certain range of viewing angles. The compensation film has the general compensation principle of correcting the phase difference generated by the liquid crystal at different viewing angles, so that the birefringent property of the liquid crystal molecules is symmetrically compensated. Different compensation films are used for different liquid crystal display modes. The compensation films used in a large-sized liquid crystal TV are mostly targeted in the VA display mode, including N-TAC of the Konica company in the early stage, and Zeonor of the OPOTES company, the F-TAC series of the Fujitsu company, X-plate of the Nitto Denko, etc. developed continuously afterwards.
FIG. 1 is an exploded drawing of the display device in the existing technology. As shown in FIG. 1, this display device includes a VA (Vertical Alignment) liquid crystal unit 300 and an optical compensation structure, the latter further including a front side polarizer 100 and a rear side polarizer 200 arranged at both sides of the VA liquid crystal unit 300. Wherein the front side polarizer 100 is arranged at the side in the light-exiting direction of the VA liquid crystal unit 300 (as shown by the arrowhead in FIG. 1; here the light-exiting direction is defined to be the direction of the backlight source oriented to the viewer), including a front side polarization unit 110 as well as a TAC film 120 arranged between the front side polarization unit 110 and the VA liquid crystal unit 300. Generally, the front side polarization unit 110 includes another TAC film as well as a PVA film arranged between this another TAC film and the TAC film 120. The rear side polarizer 200 includes a rear side polarization unit 220 as well as a rear side biaxial film 210 arranged between the rear side polarization unit 220 and the VA liquid crystal unit 300, the rear side biaxial film 210 having the function of both a delay film and a protection film. Generally, the rear side polarization unit 220 includes a TAC film as well as a PVA film arranged between this TAC film and the rear side biaxial film 210.
The above optical compensation structure includes one layer of biaxial film, and is therefore also usually called a single-layered biaxial film compensation structure. FIG. 2 is a dark-state light leakage profile of the single-layered biaxial film compensation structure. It shows the full viewing angle dark-state light leakage distribution of the single-layered biaxial film compensation structure, indicating that there is serious light leakage at phi=20°-40°, phi=140°-160°, phi=200°-220° and phi=310°-330°, i.e. there is serious dark-state light leakage at a viewing angle close to the horizontal position. However, the position of a viewer relative to the TV determines that the viewer is easier to see at a viewing angle close to the horizontal position. Therefore, the contrast and definition at the horizontal viewing angle have the biggest influence on the viewing effect.
In order to resolve the above problem, the solution commonly used at present is substituting the double-layered biaxial film compensation structure for the above single-layered biaxial film compensation structure. Still as shown in FIG. 1, in the double-layered biaxial film compensation structure, the biaxial film is substituted for the original TAC film 120, making both the front side polarizer 100 and the rear side polarizer 200 as a polarizer with the biaxial delay film.
FIG. 3 is a dark-state light leakage profile of the double-layered biaxial film compensation structure. It shows the full viewing angle dark-state light leakage distribution of the double-layered biaxial film compensation structure, indicating that there is serious light leakage at phi=30°-60°, phi=120°-150°, phi=210°-240° and phi=300°-330°, which are between the horizontal and vertical viewing angles. Generally, the contrast and definition at the horizontal viewing angle have the biggest influence on the viewing effect, while it is difficult to view at a big viewing angle (close to the vertical viewing angle), which thus has smaller influence on the viewer. It can thus be seen that using a double-layered biaxial film compensation structure can improve the viewing effect. However, although the double-layered biaxial film compensation structure improves the display effect, it will increase the cost of the optical compensation structure, which makes the display device thus produced in an inferior position in the market competition.